Vacation Savings Planner

A higher Savings Account Rate means both your Currently Saved balance and each future monthly deposit earn more compound interest before your trip, so Interest Earned covers a larger share of your Total Trip Cost. That leaves a smaller remaining amount that has to come from Monthly Savings Needed, which is why increasing the rate (holding everything else fixed) lowers the required monthly contribution.

Monthly Savings Needed rises sharply, since the same Remaining to Save amount has to be covered in far fewer deposits and there's very little time for Savings Account Rate to add meaningful interest. In the extreme, at 1 month out, Monthly Savings Needed is close to the full Remaining to Save amount in a single deposit.

The calculator's annuity formula effectively treats each Monthly Savings Needed deposit as landing at the end of its month (an "ordinary annuity"), which is the standard, slightly more conservative convention -- it earns interest for one fewer month than a deposit made at the start of the month would, so Interest Earned is a modest underestimate if you actually save earlier in each month.

Total Trip Cost is covered by three sources added together: what you've already saved (Currently Saved), what you'll actively contribute going forward (Total You Contribute), and Interest Credited to Goal -- the share of the cost your balance covers through growth rather than deposits. Those three always sum to Total Trip Cost. One scoping note: if Currently Saved compounds past Total Trip Cost on its own, Monthly Savings Needed and Total You Contribute both go to zero and Interest Credited to Goal stops at the remaining gap, which is less than the interest the account actually earns over that period.